1. Field of the Invention
The present invention relates to stackable and nestable open-top containers and, more particularly, to a container which is specifically adapted to receive perishable food items which require circulation of a cooled air flow.
Table grapes must be cooled promptly and thoroughly after harvest to maintain satisfactory quality. The grapes must be cooled immediately to (1) minimize water loss from the fruit, (2) retard the development of decay caused by fungi, and (3) reduce the rate of respiration of the fruit. Thus, immediately after harvesting, grapes are packaged in a container, or a "lug" as it is referred to in the art, and shipped to a temporary storage facility so that they may be cooled to a desirable temperature.
There are three general methods of cooling grapes in the temporary storage facility. These methods differ in the manner in which the cooling air is brought into contact with the fruit in the lug.
A first method is commonly known as conduction. In this process, cooled air is delivered to an unvented lug. Cooling of the fruit is effected strictly through the naturally occurring conduction process. The grapes which are in contact with a cold unvented liner are cooled by conduction. That fruit in turn extracts the heat from the grapes deeper in the container by the same process. No air movement is involved within the lug.
A second method is known as parallel flow cooling. In this method, cooling air is delivered by fans on the side of the storage room to palletized fruit. Two sides of each pallet are exposed to the air flow. Alternatively, the cooling air is delivered downward from ceiling jets placed between pallets with four sides exposed. The parallel-flow method may be regarded as approaching natural-convection cooling. Here, the velocity of the air along the sides of the containers causes turbulence that results in air exchange through the vents of the package.
Finally, a third and preferred method is known as forced air cooling. In this method, air is delivered directly to the fruit by establishing a pressure gradient across the lugs placed on a pallet. The forced-air method may be considered as simply forced-convection cooling.
Each of the above methods has advantages and disadvantages; however, they differ widely in the rate and effectiveness of cooling the grapes. Particularly, there is a close relationship between the cooling rate and the accessibility of the fruit to the cooling air. When the fruit itself is brought into close contact with the air such as in the forced air cooling method, the cooling time is drastically reduced.
Forced-air cooling is advantageous because the short length of the cooling period makes it possible to cool and ship fruit the same day that it is harvested and packed. When forced air cooling is used, the grapes can be super cooled to 32.degree. F. in about 2 hours. This time is critical to the storage life of the grapes and it is also imperative to reduce the bottle-neck in grape warehousing.
FIG. 5 illustrates the principle of the forced air cooling method. In the forced air cooling process, a vacuum is created on one side of a wall of pallets of grapes while cold air is pumped in from the other side of the pallets. Air pulled by the fan from the refrigeration compartment (ice, coils, spray, or packed column) is forced through the fruit packs from one side of the lug to the other before returning to the compartment.
The rate of cooling with the forced air cooling process is greatly increased over that of the parallel-flow system because, the cooling air is brought directly to the fruit in the package rather than just to the package. By setting up a pressure gradient across the package, there is a positive flow of cooling air through the container from one side to the other providing direct contact with the packed fruit.
FIGS. 6 and 7 show a forced-air cooling system in operation. In place are eight pallets each containing stacks of six containers each thereon. A plurality of pallets may be stacked one upon another on this configuration.
A vacuum is located to one side of the stacked pallet configuration. A flexible baffle or liner is used to enclose the open space between the stacked pallets from above and at the end opposite the vacuum. In this manner, a plenum chamber is formed in the open space between the separated rows of pallets.
When the vacuum is operated, air is drawn between the two rows of pallets into the plenum chamber. The pressure gradient of the forced air cooling system flows (in the direction of the arrows) across the three containers stacked in abutting relation to each other. Thus, the vacuum draws cooled air from the outer room through the three juxtaposed lugs and up and over the grapes into the plenum chamber.
2. Description of The Prior Art
Growers of table grapes currently use three different types of containers to package and ship table grapes. These known containers utilize wood, corrugated cardboard or polystyrene. There is a consensus among the growers and receivers of grapes that the table grape containers now in use could use improvement. With the increasing concern for recycling as well as the rising price of wood, the desirability of a plastic one way shipping container for table grapes manufactured with recycled material is very high.
The primary shipping container now, used is called a TKV container which is made of wooden ends and a combination of paper and thin wood for the long side walls and bottom. The wooden TKV container is a popular package because it can be stacked in a configuration three pallets high. The TKV package is also popular because it can successfully be utilized in a cold storage facility for an extended period of time.
Prior art alternatives to the wooden TKV box are wax impregnated corrugated cardboard and foam polystyrene. The corrugated cardboard box is a short term shipping container which is used in applications where the grapes are usually picked and shipped within a period of one week. However, corrugated cardboard has a tendency to absorb moisture and fall apart. In addition, corrugated cardboard cannot be stacked in a configuration three pallets high because of limitations on the strength of the corrugated cardboard. Similarly, the polystyrene box does not stack three pallets high and has recycling limitations.
As discussed above, the specific construction of the container used to ship grapes is important to successful fruit harvesting, cooling, storage and shipping. Moreover, when using the preferred forced air cooling process, the design of the container used to hold the grapes is critical. Air that bypasses the fruit pack has little cooling effect and therefore does little to reduce the length of the required cooling period. In addition, even relatively small openings around the packages can increase significantly the fan capacity required to maintain a given static-pressure difference.
For example, prior art TKV containers have been used to store grapes temporarily during the forced air cooling process. However, when TKV containers are used, spacers or cleats are inserted between juxtaposed containers which are stacked on top of one another. Also, there are cleats on the lids, necessary for attachment. With this arrangement, a substantial amount of the cooling air flow is lost between the stacked TKV containers. Because the cooling air flow directed at the containers will follow the path of least resistance, a large quantity cooling air naturally flows between the containers into the open areas created by the spacers or cleats, as shown, for example, in FIG. 1A. Accordingly, because a large quantity of air is lost, the volume of cooling air required to maintain a given static-pressure difference is significantly increased. In turn, the large increase in required air volume necessitates a great increase in fan capacity to cool a given quantity of fruit. Since more power is needed, the cost is greater.
Thus, ideally and for maximum efficiency in a forced-air system, the only air that should be permitted to pass through the pallet of containers is that which comes into direct contact with the fruit. Ideally there would be no air gap between juxtaposed containers which will be used in forced air cooling. There is thus a need for a stackable and nestable container which is suitable for storing perishable food items and which minimizes the detrimental air flow loss when the container is subjected to a forced air cooling process.